The primary goal of this project was to design and implement a multi-institutional, open-label phase I/II study of hydroxyurea (HU) therapy in children with sickle cell disease. To achieve this main goal, the project had four specific aims: 1) To determine if HU would elevate the concentration of fetal hemoglobin, hematocrit and red cell mean corpuscular volume in children with sickle cell anemia between the ages of 5 and 18 years. 2) To determine if the therapeutic dose of HU in pediatric patients was similar to that in adults. 3) To determine if the hematologic and renal toxicities in children were similar to those in adults. 4) To determine if there were adverse effects on growth in children taking HU.